Multilayer, transparent, sealable, biaxially oriented polyester film, its use and process for its production

ABSTRACT

The invention relates to a biaxially oriented, sealable polyester film with at least one base layer (B), with a sealable outer layer (A) and with another, non-sealable outer layer (C). The sealable outer layer (A) has a minimum sealing temperature (fin) of not more than 110° C. and a seal seam strength (fin) of at least 1.3 N/15 mm of film width. The two outer layers (A) and (C) are characterized by particular features. The film of the invention is particularly suitable for use in flexible packaging, and particularly and specifically for use on systems which operate in a vacuum.

The invention relates to a transparent, sealable, coextruded, biaxiallyoriented polyester film composed of at least one base layer (B) and ofouter layers (A) and (C) applied to the two sides of this base layer.The two outer layers (A) and (C) are characterized by certain, inventivefeatures. The invention further relates to the use of the film and to aprocess for its production.

BACKGROUND OF THE INVENTION

Sealable, biaxially oriented polyester films are known. These knownfilms either have good sealing performance or good optical properties,or acceptable processing performance.

GB-A 1 465 973 describes a coextruded, two-layer polyester film in whichone layer is composed of isophthalic acid-containing and terephthalicacid-containing copolyesters and in which the other layer is composed ofpolyethylene terephthalate. The specification gives no usefulinformation concerning the sealing performance of the film. Lack ofpigmentation means that the process for producing the film is notreliable (the film cannot be wound) and that there are severelimitations on further processing of the film. The film can certainlynot be further processed on systems which operate using a vacuum(metalization, coating with ceramic materials, etc.).

EP-A-0 035 835 describes a coextruded sealable polyester film which hasparticles admixed in the sealable layer to improve winding andprocessing performance, the median size of these particles exceeding thethickness of the sealable layer. The particulate additives form surfaceprotrusions which prevent undesired blocking and sticking to rollers orguides. No further detail is given concerning incorporation ofantiblocking agents in the other, non-sealable layer of the film. If noantiblocking agents are present in this layer, the film has restrictedfurther processability, and especially has restricted capability forprocessing in a vacuum. The preferred use of bimodal particle systemsmakes the distribution of particle diameter very broad. This, togetherwith the concentrations stated in the examples of the specification,impairs the sealing performance of the film. The specification gives noinformation on the sealing temperature range, in particular on theminimum sealing temperature (MST) of the film. Seal seam strength ismeasured at 140° C. and is in the range from 85 to 120 N/m(corresponding to from 1.275 to 1.8 N/15 m of film width). In addition,the film has unsatisfactory optical properties. The haze of the film isin the range from 5 to 15%, and this is markedly too high for mostapplications.

EP-0 379 190 discloses a coextruded polyester film which has a sealablelayer whose sealing energy is greater than 400 g·cm/15 mm. The sealablelayer comprises inorganic, or else additionally organic particles, addedat a concentration of from 0.01 to 5% to the sealable layer. Thediameter of these particles is smaller than the thickness of thesealable layer, but no definitive information is given on the absolutesize of the particles. In the examples, the diameter varies from 2 to3.5 μm. In the preferred embodiment, the particles are monodispersed andalmost spherical. The base layer may likewise comprise particulateadditives of this type. However, that specification gives no teachingconcerning, for example, how the base is to be pigmented in order tomake the sealable film reliably processable on systems in a vacuum. Thecapability of the film for, for example, vacuum-coating is thereforenon-existent or only very restricted.

EP-A-0 432 886 describes a coextruded multilayer polyester film whichhas a first surface on which a sealable layer has been arranged and asecond surface on which an acrylate layer has been arranged. Here, too,the sealable outer layer may be composed of isophthalic acid-containingand terephthalic acid-containing copolyesters. The reverse-side coatinggives the film improved processing performance. The specification givesno information concerning the sealing range of the film. Seal seamstrength is measured at 140° C. For a sealing layer of thickness 11 μmthe seal seam strength given is 761.5 N/m (corresponding to 11.4 N/15 mmof film width). A disadvantage of the reverse-side acrylate coating isthat this side does not seal with respect to the sealable outer layer.This gives the film a limited field of application.

EP-A-0 515 096 describes a coextruded, multilayer sealable polyesterfilm which comprises an additional additive on the sealable layer, andwhich has appropriate topography (number of elevations and range for theaverage height of the elevations). The additive may comprise inorganicparticles, for example, and is preferably applied in an aqueous layer tothe film during its production. Furthermore, the additive (=antiblockingagent) may also be directly incorporated into the sealable layer. Theresult is said to be that the film retains good sealing properties andprocesses well. The reverse side of the film preferably comprises onlyvery few, relatively small, particles, which pass into this layer mainlyvia the regrind. The specification gives no information concerning thesealing temperature range of the film. Seal seam strength is measured at140° C. and is more than 200 N/m (corresponding to 3 N/15 mm of filmwidth). For a sealing layer of thickness 3 μm the seal seam strengthgiven is 275 N/m (corresponding to 4.125 N/15 mm of film width). Thefilm has only limited suitability for further processing on systemswhich operate in a vacuum (physical vapor deposition (PVD) processes,such as metalization and coating with ceramic substances). The reasonsfor this are the low roughness of the hot-sealable layer, the lowaverage height of the elevations of the particles present in this layer,and the provision of only a low concentration (e.g. via the regrind) ofantiblocking agents in the other, non-sealable layer of the film. Inaddition, the optical properties of the film are unsatisfactory. Thehaze of the film is greater than 3%, and this is too high for manyapplications.

WO 98/06575 describes a coextruded multilayer polyester film whichcomprises a sealable outer layer and a non-sealable base layer. The baselayer here may be composed of one or more layers, one of the layersbeing in contact with the sealable layer. The other (exterior) layerthen forms the second non-sealable outer layer. Here, too, the sealableouter layer may be composed of isophthalic acid-containing andterephthalic acid-containing copolyesters, but no antiblocking particlesare present in these. In addition, the film also comprises at least oneUV absorber, which is added to the base layer in a weight ratio of from0.1 to 10%. The base layer of this film has conventional antiblockingagents. The film has good sealability, but is relatively unsuitable formetalization. In addition, it has shortcomings in optical properties(gloss and haze).

EP-A 1 138 480 describes a coextruded, biaxially oriented, sealablepolyester film with at least one base layer (B), with a sealable outerlayer (A), and with another outer layer (C). The sealable outer layer(A) has a minimum sealing temperature of not more than 110° C., and aseal seam strength of at least 1.3 N/15 mm of film width, and is alsocharacterized by appropriate features with regard to the topography ofthe two outer layers (A) and (C). The relatively smooth sealable layer,which in comparison comprises only relatively few antiblockingparticles, has excellent sealing properties and is easy to produce, andhas good processability (printing, cutting, laminating, etc.) It has no,or only very restricted, capability for use on systems which operate ina vacuum.

It was therefore an object of the present invention to provide atransparent, coextruded, sealable, biaxially oriented polyester filmwhich does not have the disadvantages of the prior-art films mentioned,and which in particular has very good processability on systems whichoperate in a vacuum (metalization, coating with ceramic substances, andin general the use of PVD processes and chemical vapor deposition (CVD)processes), and which has very good optical properties. For example, thefilm is intended not to block during coating with ceramic substances(e.g. SiO_(x), Al₂O₃) in a vacuum, and is intended to give good windingperformance in those processes. This applies both to the unwinding ofthe film prior to the coating process and also to the winding-up of thefilm after the coating process. Blocking of the two surfaces of the filmafter the coating process can cause the (sealable) reverse side of thefilm to “stick” to the coated (non-sealable) side, and this isundesirable. On rewinding of the blocked roll (where the blocking sitesmostly have random distribution), the material can be completely tornaway from the film surface and transferred to the other, sealable filmsurface, and this is likewise undesirable. In addition, the blocking ofthe two surfaces of the film during the subsequent steps of processingcan lead to break-off of the film web. A further object of the presentinvention is to improve the processability of the film in processes inwhich at least one of the two surfaces is coated in such a way that thisside becomes duller after coating. This means that the coefficient offriction of that side increases, giving that side of the film asusceptibility to blocking. This may occur, for example, whenscratch-resistant coatings (acrylates, epoxy resins) are applied. Thefilm is also intended to have good sealability, the intention here beingthat the sealable layer (A) be sealable with respect to itself (finsealing) and also with respect to the reverse side (non-sealable side(C)) (lap sealing). A further intention is to ensure that cut materialarising during production of the film can be returned as regrind to theproduction process in amounts of up to 60% by weight, based on the totalweight of the film, without any significant resultant adverse effect onthe physical and optical properties of the film. In summary, theproperties which were in particular to be improved over prior-art filmswere the following:

-   the properties needed for good processing of the film in a vacuum-   winding, in particular during processing of the film in a vacuum-   blocking performance of the two surfaces of the film,-   the optical properties of the film, in particular haze and gloss.

BRIEF DESCRIPTION OF THE INVENTION

According to the invention, the object is achieved by providing atransparent, coextruded, biaxially oriented, sealable polyester filmwith at least one base layer (B), with a sealable outer layer (A), andwith another outer layer (C), where the sealable outer layer (A) has aminimum sealing temperature (fin) of not more than 110° C. and a sealseam strength (fin) of at least 1.3 N/15 mm of film width, and where thetwo outer layers (A) and (C) are characterized by the followingfeatures:

The sealable outer layer (A):

-   a) comprises particles composed of synthetic SiO₂ with a median    particle diameter d₅₀ of from 2.5 to 10 μm,-   b) comprises particles whose diameter has a SPAN98 which is less    than or equal to 1.80,-   c) comprises particles at a concentration of from 500 to 5000 ppm,-   d) has roughness R_(a) greater than 60 nm,-   e) has a side A/side A coefficient of friction smaller than 0.8.

The non-sealable metalized outer layer (C):

-   f) comprises particles composed of synthetic SiO₂ with a median    particle diameter d₅₀ of from 1.5 to 6 μm,-   g) comprises particles whose diameter has a SPAN98 less than or    equal to 2.0,-   h) comprises particles at a concentration of from 1000 to 5000 ppm,-   i) has roughness R_(a) such that 30≦R_(a)≦150 nm,-   j) has a side C/side C coefficient of friction smaller than 0.6.

BRIEF DESCRIPTION OF FIGS.

FIGS. 1 and 2 are graphs illustrating the methods of calculating themedian particle diameter d₅₀ and the SPAN 98, respectively and

FIG. 3 is a schematic illustrating the method of testing seal seamstrength.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, the film has at least three layers, thelayers then present therein being the base layer (B), the sealable outerlayer (A), and the non-sealable outer layer (C).

The base layer (B) of the film is preferably composed of at least 80% byweight of a thermoplastic polyester. Polyesters suitable for thispurpose are those made from ethylene glycol and terephthalic acid(=polyethylene terephthalate, PET), from ethylene glycol andnaphthalene-2,6-dicarboxylic acid (=polyethylene 2,6-naphthalate, PEN),from 1,4-bishydroxymethylcyclohexane and terephthalic acid(=poly-1,4-cyclohexanedimethylene terephthalate, PCDT), or else madefrom ethylene glycol, naphthalene-2,6-dicarboxylic acid andbiphenyl-4,4′-dicarboxylic acid (=polyethylene 2,6-naphthalatebibenzoate, PENBB). Particular preference is given to polyesters atleast 90 mol %, preferably at least 95 mol %, of which is composed ofethylene glycol units and terephthalic acid units, or of ethylene glycolunits and naphthalene-2,6-dicarboxylic acid units. Preference is alsogiven to mixtures (blends) composed of the abovementioned polymers, inparticular blends which comprise polyethylene terephthalate andpolyethylene 2,6-naphthalate. Among these, very particular preference isin turn given to those blends which comprise the abovementioned polymersand have semicrystalline character. The remaining monomer units derivefrom those other aliphatic, cycloaliphatic or aromatic diols and,respectively, dicarboxylic acids which can also be present in layer (A)or in layer (C).

Other examples of suitable aliphatic diols are diethylene glycol,triethylene glycol, aliphatic glycols of the formula HO—(CH₂)_(n)—OH,where n is an integer from 3 to 6 (in particular 1,3-propanediol,1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol) and branchedaliphatic glycols having up to 6 carbon atoms. Among the cycloaliphaticdiols, mention should be made of cyclohexanediols (in particular1,4-cyclohexanediol). Examples of other suitable aromatic diols have theformula HO—C₆H₄—X—C₆H₄—OH, where X is —CH₂—, —C(CH₃)₂—, —C(CF₃)₂, —O—,—S— or —SO₂—. Bisphenols of the formula HO—C₆H₄—C₆H₄—OH are also verysuitable.

Other aromatic dicarboxylic acids are preferably benzenedicarboxylicacids, naphthalenedicarboxylic acids, such as naphthalene-1,4- or-1,6-dicarboxylic acid, biphenyl-x,x′-dicarboxylic acids, in particularbiphenyl-4,4′-dicarboxylic acid, diphenylacetylene-x,x′-dicarboxylicacids, in particular diphenylacetylene-4,4′-dicarboxylic acid, orstilbene-x,x′-dicarboxylic acids. Among the cycloaliphatic dicarboxylicacids mention should be made of cyclohexanedicarboxylic acids, inparticular cyclohexane-1,4-dicarboxylic acid. Among the aliphaticdicarboxylic acids, the C₃-C₁₉ alkanediacids are particularly suitable,and the alkane moiety here may be straight-chain or branched.

One way of preparing the polyesters is the transesterification process.Here, the starting materials are dicarboxylic esters and diols, whichare reacted using the customary transesterification catalysts, such asthe salts of zinc, of calcium, of lithium, of magnesium or of manganese.The intermediates are then polycondensed in the presence of well-knownpolycondensation catalysts, such as antimony trioxide or titanium salts.Another equally good preparation method is the direct esterificationprocess in the presence of polycondensation catalysts. This startsdirectly from the dicarboxylic acids and the diols.

The sealable outer layer (A) coextruded onto the base layer (B) has astructure based on polyester copolymers and is substantially composed ofcopolyesters predominantly made of isophthalic acid units and ofterephthalic acid units and of ethylene glycol units. The other monomerunits derive from those other aliphatic, cycloaliphatic, or aromaticdiols and, respectively, dicarboxylic acids which can also be present inthe base layer. Preferred copolyesters which provide the desired sealingproperties are those composed of ethylene terephthalate units and ofethylene isophthalate units, and of ethylene glycol units. Theproportion of ethylene terephthalate is from 40 to 95 mol % and thecorresponding portion of ethylene isophthalate is from 60 to 5 mol %.Preference is given to copolyesters where the proportion of ethyleneterephthalate is from 50 to 85 mol % and the corresponding proportion ofethylene isophthalate is from 50 to 15 mol %, and highly preferredcopolyesters are those where the proportion of ethylene terephthalate isfrom 60 to 80 mol % and the corresponding proportion of ethyleneisophthalate is from 40 to 20 mol %.

For the other, non-sealable outer layer (C), or for any intermediatelayers present, use is in principle made of those polymers describedabove for the base layer (B).

The desired sealing properties and the desired processing properties, inparticular the desired performance of the film of the invention duringcoating in a vacuum, are obtained from the combination of the propertiesof the copolyesters used for the sealable outer layer and from thetopographies of the sealable outer layer (A) and the non-sealable outerlayer (C).

The minimum sealing temperature (fin) of 110° C. and the sealed seamstrength (fin) of at least 1.3 N/15 mm of film width are achieved, forexample, if the copolymers described in greater detail above are usedfor the sealable outer layer (A). The best sealing properties of thefilm are obtained if no other additives are added to the copolymer, inparticular no inorganic or organic particles. However, in that case thehandling properties of the film are poor, because the surface of thesealable outer layer (A) is highly susceptible to blocking. The filmdoes not give good winding and is not very suitable for furtherprocessing on high-speed packaging machinery. A film of this type iscompletely unsuitable for processing on systems which operate with theaid of an applied vacuum. Due to the vacuum prevailing, the film isparticularly susceptible to blocking. The individual plies of film arepressed onto one another by the vacuum (below 10⁻² Torr) and “stick”immediately if they do not have sufficient pigmentation.

To improve the handling properties of the film, and the processabilityof the film on systems which utilize a vacuum, the sealable outer layer(A) has to be modified. This is best achieved with the aid of suitableantiblocking agents of selected size, a certain amount of these beingadded to the sealable layer, and specifically in such a way as firstlyto minimize the blocking of the film and secondly to avoid substantialimpairment of sealing properties. The desired properties of the filmwith respect to sealability and processability in a vacuum can beachieved if the sealable outer layer (A) is characterized by thefollowing set of parameters:

-   a) According to the invention, the sealable outer layer (A)    comprises particles (=antiblocking agents) composed of synthetic    SiO₂ with a median particle diameter d₅₀ of from 2.5 to 10 μm.    -   Typical known antiblocking agents (also termed pigments) are        inorganic and/or organic particles, such as calcium carbonate,        amorphous silicate, talc, magnesium carbonate, barium carbonate,        calcium sulfate, barium sulfate, lithium phosphate, calcium        phosphate, magnesium phosphate, aluminum oxide, lithium        fluoride, the calcium, barium, zinc, or manganese salts of the        dicarboxylic acids used, carbon black, titanium dioxide, kaolin        or crosslinked polystyrene particles, or crosslinked acrylate        particles. The particles may be added to each of the layers in        the respective advantageous concentrations, e.g. in the form of        a glycolic dispersion during the polycondensation process, or by        way of masterbatches during the extrusion process.    -   According to the invention, however, these abovementioned        antiblocking agents (except the amorphous silicate) are not        preferred. Particles with spherical shape are likewise given low        preference. These (spherical) particles have a tendency toward        undesirable release from the film (chalking) during processing        of the film, which implies exposure of the film to high        mechanical load.    -   According to the invention, preferred particles are        synthetically prepared, amorphous SiO₂ particles in colloidal        form. These particles become extremely well bound into the        polymer matrix and create only a few vacuoles (cavities).    -   Vacuoles arise at the particles during the biaxial orientation        process, and generally cause haze, and are therefore not very        suitable for the present invention. The (synthetic) preparation        of the SiO₂ particles (also termed silica gel) begins by mixing        sulfuric acid and sodium silicate with one another under        controlled conditions to form hydrosol. This finally forms a        hard, transparent mass known as hydrogel. Once the sodium        sulfate produced as by-product has been removed by washing, it        can be dried and further processed. The important physical        parameters, e.g. pore volume, pore size, and the dimensions of        the surface area of the resultant silica gel, may be varied via        control of the pH of the washing water and of the drying        conditions. The desired particle size (e.g. the d₅₀ value) and        the desired particle size distribution (e.g. the SPAN98) are        obtained by suitable milling of the silica gel (e.g.        mechanically or hydromechanically). Examples of producers of        these particles are the companies Grace, Fuji, Degussa, and        Crosfield.    -   It has proven particularly advantageous to use particles with a        median particle diameter d₅₀ of from 2.5 to 10 μm, preferably        from 2.8 to 8 μm, and particularly preferably from 3.1 to 6 μm.        If particles with a diameter below 2.5 μm are used, haze        increases (at comparable concentrations) and the susceptibility        of the film to blocking becomes greater. Particles with diameter        greater than 10 μm generally cause filter problems.-   b) According to the invention, the sealable outer layer (A)    comprises particles whose diameter has a scattering described by    SPAN98≦1.8 (SPAN98 as defined in test specification). SPAN98 is    preferably ≦1.7 and particularly preferably ≦1.6. If, in contrast,    the outer layer (A) of the film comprises a particle system in which    the SPAN98 of the diameter is greater than 1.8, the optical    properties and the sealing properties of the film become poorer.-   c) According to the invention, the sealable outer layer (A)    comprises particles at a concentration of from 500 to 5000 ppm. The    concentration of the particles is preferably from 800 to 4000 ppm,    and particularly preferably from 1000 to 3000 ppm. If, in contrast,    the outer layer (A) of the film comprises a particle system in which    the particles are present at a concentration of less than 500 ppm,    it is less suitable in particular for processing in a vacuum. If, in    contrast, the outer layer (A) of the film comprises a particle    system in which the particles are present at a concentration of more    than 5000 ppm, the haze of the film becomes too great.-   d) According to the invention, the roughness of the sealable outer    layer, expressed via the R_(a) value, is greater than 60 nm. The    roughness R_(a) is preferably greater than 80 nm, and particularly    preferably greater than 100 nm. Otherwise, the film is less suitable    for processing in a vacuum.-   e) According to the invention, the sealable outer layer (A) has a    side A/side A coefficient of friction smaller than 0.8. Preference    is given to a coefficient of friction smaller than 0.75, and    particular preference is given to a coefficient of friction smaller    than 0.7. Otherwise, the film is less suitable for processing in a    vacuum.

For further improvement in processing performance, in particular in theprocessing of the sealable film in a vacuum, the non-sealable outerlayer (C) should be characterized by the following set of parameters:

-   a) According to the invention, the non-sealable outer layer (C)    comprises particles composed of synthetically prepared, amorphous    SiO₂ with a median particle diameter d₅₀ of from 1.5 to 6 μm. To    achieve the object, it has proven particularly advantageous and    preferable to use particles with a median particle diameter d₅₀ of    from 2.0 to 5 μm, particularly preferably from 2.5 to 4 μm.-   b) According to the invention, the non-sealable outer layer (C)    comprises particles whose diameter has a scatter described by    SPAN98≦2.0. Preference is given to SPAN98≦1.9, and particular    preference is given to SPAN98≦1.8. If, in contrast, the outer    layer (C) of the film comprises a particle system in which the    SPAN98 of the diameter is greater than 2.0, the optical properties    of the film become poorer.-   c) According to the invention, the non-sealable outer layer (C)    comprises particles at a concentration of from 1000 to 5000 ppm. The    concentration of the particles is preferably from 1200 to 4000 ppm,    and particularly preferably from 1500 to 3000 ppm. If, in contrast,    the outer layer (C) of the film comprises a particle system in which    the particles are present at a concentration of less than 1000 ppm,    it is less suitable for processing in a vacuum. If, in contrast, the    outer layer (C) of the film comprises a particle system in which the    particles are present at a concentration of more than 5000 ppm, the    haze of the film becomes too great.-   d) According to the invention, the roughness of the non-sealable    outer layer, expressed via the R_(a) value, is such that    30≦R_(a)≦150 nm. Roughness R_(a) is preferably such that    35≦R_(a)≦130 nm, and particularly preferably 40≦R_(a)≦110 nm.    Otherwise, this surface is less suitable for processing in a vacuum.-   e) According to the invention, the non-sealable outer layer (C) has    a side C/side C coefficient of friction smaller than 0.6. Preference    is given to a coefficient of friction smaller than 0.55, and    particular preference is given to a coefficient of friction smaller    than 0.5. Otherwise, the film is less suitable for processing in a    vacuum.

To achieve the abovementioned properties of the sealable film, it hasproven advantageous for the amount of particles in the base layer (B) tobe set lower than in the two outer layers (A) and (C). In thethree-layer film of the abovementioned type, the amount of particles inthe base layer (B) is advantageously to be from 0 to 0.15% by weight,preferably from 0 to 0.12% by weight, in particular from 0 to 0.10% byweight. It has proven particularly advantageous for the particlesincorporated into the base layer to be only those which pass into thefilm by way of regrind of the same type of material. The opticalproperties of the film, in particular the haze of the film, are thenparticularly good.

In the preferred form, the film is composed of three layers: the baselayer (B) and outer layers (A) and (C) applied to the two sides of thisbase layer, the outer layer (A) being sealable with respect itself (finsealing) and with respect to the outer layer (C) (lap sealing). The lapsealing here is generally somewhat poorer than the fin sealing. Theminimum sealing temperature or lap sealing should not be above 120° C.,and the seal seam strength should be at least 1.0 N/15 mm of film width.This is the case in the films of the invention.

Between the base layer and the outer layers there may, whereappropriate, also be an intermediate layer. This may again be composedof the polymers described for the base layers. In one particularlypreferred embodiment, the intermediate layer is composed of thepolyesters used for the base layer. The intermediate layer may alsocomprise the conventional additives described below. The thickness ofthe intermediate layer is generally greater than 0.3 μm and ispreferably in the range from 0.5 to 15 μm, particularly in the rangefrom 1.0 to 10 μm, and particularly preferably in the range from 1.0 to5 μm.

In the particularly advantageous three-layer embodiment of the film ofthe invention, the thickness of the outer layers (A) and (C) isgenerally greater than 0.5 μm and is preferably in the range from 0.55to 6.0 μm, particularly preferably in the range from 0.6 to 5 μm, inparticular in the range from 0.65 to 4 μm, and very particularlypreferably in the range from 0.7 to 3 μm, and the thicknesses of theouter layers (A) and (C) here may be identical or different.

The total thickness of the polyester film of the invention may varywithin certain limits. It is from 3 to 100 μm, in particular from 4 to80 μm, preferably from 5 to 60 μm, the proportion made up by the layer(B) preferably being from 45 to 90% of the total thickness.

The base layer and the other layers may also comprise conventionaladditives, such as stabilizers and other fillers. Their addition to thepolymer or polymer mixture advantageously takes place prior to melting.Examples of stabilizers used are phosphorus compounds, such asphosphoric acid or phosphoric esters.

The invention also provides a process for producing the polyester filmof the invention by the coextrusion process known from the literature.

To produce the film, the polymers for the base layer (B) and the twoouter layers (A) and (C) are fed to three separate extruders. Anyforeign bodies or contamination present may be removed from the polymermelt by suitable filters prior to extrusion. The melts corresponding tothe individual layers (A), (B), and (C) of the film are then coextrudedthrough a flat-film die, and the resultant film is drawn off on one ormore rollers for solidication, the film is then biaxially stretched(oriented), the biaxially stretched film is heat-set and, whereappropriate, corona- or flame-treated on the surface intended fortreatment.

As usual in the coextrusion process, the polymer or the polymer mixturesfor the individual layers is/are first compressed and plastified in therespective extruders, and any additives provided may by this stage bepresent in the polymer or in the polymer mixture. The melts are thensimultaneously extruded through a flat-film die (slot die), and theextruded multilayer melt is drawn off on one or more take-off rollers,whereupon the melt cools and solidifies to give a prefilm.

The biaxial orientation is generally carried out sequentially. In theprocess, the prefilm is preferably stretched first longitudinally (i.e.in the machine direction=MD) and then transversely (i.e. perpendicularlyto the machine direction=TD). This gives orientation of the polymerchains. The longitudinal stretching can be carried out, for example,with the aid of two rolls running at different speeds corresponding tothe desired stretching ratio. For the transverse stretching use isgenerally made of an appropriate tenter frame, by clamping both edges ofthe film and then subjecting it to bilateral tension at an elevatedtemperature.

The temperature at which the orientation is carried out may vary over arelatively wide range and depends on the film properties desired. Thelongitudinal stretching is generally carried out at from about 80 to130° C., and the transverse stretching at from about 90 to 150° C. Thelongitudinal stretching ratio is generally in the range from 2.5:1 to6:1, preferably from 3:1 to 5.5:1. The transverse stretching ratio isgenerally in the range from 3.0:1 to 5.0:1, preferably from 3.5:1 to4.5:1. Prior to the transverse stretching, one or both surfaces of thefilm may be in-line coated by known processes. The in-line coating mayserve, for example, to improve adhesion of a metal layer or of anyprinting ink subsequently to be applied, or else to improve antistaticperformance or processing performance.

In the heat-setting which follows, the film is held for from about 0.1to 10 s at a temperature of from 150 to 250° C. The film is then woundup in a conventional manner.

One or both surface(s) of the film is/are preferably corona- orflame-treated by one of the known methods after biaxial stretching. Theintensity of treatment is generally in the range above 50 mN/m.

The film may also be coated in order to establish other desirableproperties. Typical coatings have adhesion-promoting, antistatic,slip-improving, or release action. Clearly, these additional layers maybe applied to the film via in-line coating by means of an aqueousdispersion prior to the stretching step in a transverse direction.

The film of the invention has excellent sealability, very good handlingproperties, and very good processing performance, in particular duringprocessing in a vacuum. The sealable outer layer (A) of the film sealsnot only with respect to itself (fin sealing), but also with respect tothe non-sealable outer layer (C) (lap sealing).

In addition, it was possible to make a marked improvement in the haze ofthe film. The haze of the film of the present invention is less than3.0%, preferably less than 2.7%, and particularly preferably less than2.5%. Furthermore, it has been ensured that cut material (regrind) canbe reintroduced to the extrusion process during production of the filmin amounts in the range from 10 to 60% by weight, based on the totalweight of the film, without any significant adverse effect on thephysical properties of the film, in particular on its appearance.

The film therefore has excellent suitability for use in flexiblepackaging, and indeed particularly wherever its excellent sealingproperties and its good processability are of great importance. Thisapplies in particular to coating in a vacuum.

The following table (table 1) gives the most important film propertiesof the invention again at a glance.

TABLE 1 Range of the Particularly invention Preferred preferred UnitTest method Outer layer A Minimum sealing temperature <110 <108 <105 °C. internal (fin sealing) Minimum sealing temperature <120 <118 <115 °C. internal (lap sealing) Seal seam strength (fin sealing)≧1.3 >1.4  >1.5 N/15 mm internal Seal seam strength (lap sealing)≧1.0 >1.1  >1.2 N/15 mm internal Outer layer thickness >0.5 0.55-6 0.6-5μm Particle diameter d₅₀ 2.5-10 2.8-8 3.1-6 μm internal SPAN98scattering ≦1.8 ≦1.7  ≦1.6 — internal Filler concentration  500-5000 800-4000 1000-3000 ppm internal Average roughness R_(a) >60 >80  >100nm DIN 4768, cut-off at 0.25 mm A/A COF <0.8 <0.75 <0.7 DIN 53375 Gloss,20° >120 >130 >140 DIN 67530 Outer layer C Outer layer thickness >0.50.55-6   0.6-5 μm Particle diameter d₅₀ 1.5-6 2.0-5   2.5-4 μm internalSPAN98 scattering ≦2.0 ≦1.9  ≦1.8 — internal Filler concentration1000-5000 1200-4000 1500-3000 ppm internal Average roughness R_(a) 30-150  35-130  40-110 nm DIN 4768, Cut off at 0.25 mm C/C COF <0.6<0.55 <0.5 DIN 53375 Gloss 20° >120 >130 >140 DIN67530 Other filmproperties Haze <3.0 <2.7  <2.5 % ASTM D1003-52

For the purposes of the present invention, the following test methodswere utilized to characterize the raw materials and the films:

Measurement of Median Diameter d₅₀

Median diameter d₅₀ was determined by laser scanning on a MalvernMastersizer (examples of other test equipment being the Horiba LA 500 orSympathec Helos, which use the same principle of measurement). For this,the specimens were placed in a cell with water and this was theninserted into the test equipment. The dispersion is scanned by thelaser, and a particle size distribution is determined from the signal bycomparison with a calibration curve. The particle size distribution hastwo characterizing parameters, the median value d₅₀ (=a measure of theposition of the median) and the scatter, termed the SPAN98 (=a measureof the scattering of particle diameter). The test procedure isautomatic, and also includes the mathematical determination of the d₅₀value. The d₅₀ value is determined here in accordance with itsdefinition from the (relative) cumulative particle size distributioncurve: the point of intersection of the 50% ordinate value with thecumulative curve gives the desired d₅₀ value (also termed median, cf.FIG. 1) on the abscissa axis.

Measurement of SPAN98

The test equipment used to determine scatter, SPAN98, was the same asthat described above for the determination of median diameter d₅₀.SPAN98 is defined here as follows:${{SPAN}\quad 98} = {\frac{d_{98} - d_{10}}{d_{50}}.}$

Determination of d₉₈ and d₁₀ is in turn based on the (relative)cumulative particle size distribution curve. The point of intersectionof the 98% ordinate value with the cumulative curve directly gives thedesired d₉₈ value on the abscissa axis, and the point of intersection ofthe 10% ordinate value of the cumulative curve with the curve gives thedesired d₁₀ value on the abscissa axis (cf. FIG. 2).

SV (Standard Viscosity)

Standard viscosity SV (DCA) is measured in dichloroacetic acid by amethod based on DIN 53726.

Seal Seam Strength

To determine seal seam strength, two pieces of film (1) (100 mm×100 mm)are placed one on top of the other and sealed at 130° C. with a sealingtime of 0.5 s and a sealing pressure of 2 bar (HSG/ET sealer fromBrugger, bilaterally heated sealing jaws, seal seam (2) (10 mm×100 mm).Test strips of width 15 mm are cut from the sealed specimens. The T-sealseam strength, i.e. the force (3) needed to separate the test strips, isdetermined using a tensile testing machine (e.g. Zwick) at a separationvelocity of 200 mm/min., the plane of the seal seam being at rightangles to the direction of tension [cf. FIG. 3; with claw (4)]. Sealseam strength is given in N per 15 mm film strip (e.g. 3 N/15 mm).

Determination of Minimum Sealing Temperature

Hot-sealed specimens (seal seam 10 mm×100 mm) are produced as describedabove under measurement of seal seam strength using the Brugger HSG/ETsealer apparatus, by sealing the film at different temperatures with theaid of two heated sealing jaws at a sealing pressure of 2 bar and with asealing time of 0.5 s. From the sealed specimens, test strips of 15 mmwidth were cut. The minimum sealing temperature is the temperature atwhich a seal seam strength of at least 0.5 N/15 mm is achieved. Sealseam strength was measured as in the determination of seal seamstrength.

Roughness

Roughness R_(a) of the film was determined to DIN 4768 with a cut-off of0.25 mm. This measurement was not made on a glass plate but in a ring.In the ring method, the film is clamped into a ring in such a way thatneither of the two surfaces is in contact with a third surface (e.g.glass).

Coefficient of Friction (COF)

Coefficient of friction was determined to DIN 53375. Coefficient ofsliding friction was measured 14 days after production.

Surface Tension

Surface tension was determined by what is known as the ink method (DIN53 364).

Haze

Hölz haze was measured by a method based on ASTM D1003-52 but, in orderto utilize the most effective measurement range, measurements were madeon four pieces of film laid one on top of the other, and a 1° slitdiaphragm was used instead of a 4° pinhole.

Gloss

Gloss was determined to DIN 67 530. Reflectance was measured, this beingan optical value characteristic of a film surface. Based on thestandards ASTM D523-78 and ISO 2813, the angle of incidence was set at20°. A beam of light hits the flat test surface at the set angle ofincidence and is reflected and/or scattered thereby. A proportionalelectrical variable is displayed representing light rays hitting thephotoelectronic detector. The value measured is dimensionless and mustbe stated together with the angle of incidence.

The invention is further illustrated in the following examples.

EXAMPLE 1

Chips made from polyethylene terephthalate (produced by thetransesterification process using Mn as transesterification catalyst, Mnconcentration: 100 ppm) were dried at 150° C. to residual moisture below100 ppm, and fed to the extruder for the base layer (B). Similarly,chips made from polyethylene terephthalate and particles were fed to theextruder for the non-sealable outer layer (C).

In addition to this, chips were produced from a linear polyestercomposed of an amorphous copolyester comprising 78 mol % of ethyleneterephthalate and 22 mol % of ethylene isophthalate (prepared by thetransesterification process using Mn as transesterification catalyst, Mnconcentration: 100 ppm). The copolyester was dried at a temperature of100° C. to residual moisture below 200 ppm and fed to the extruder forthe sealable outer layer (A).

Coextrusion followed by stepwise longitudinal and transverse orientationwas then used to produce a transparent, three-layer film with ABCstructure and a total thickness of 12 μm. The thickness of each of theouter layers is found in table 2.

Outer layer (A) was a mixture of:

-   96.0% by weight of copolyester with SV of 800-   4.0% by weight of Masterbatch made from 95% by weight of copolyester    (SV of 800) and 5.0% by weight of ®Sylysia 430 (synthetic SiO₂ from    Fuji, Japan); SPAN98=1.7

Base layer (B):

-   100.0% by weight of polyethylene terephthalate with SV of 800-   Outer layer (C) was a mixture of:-   85% by weight of polyethylene terephthalate with SV of 800-   15% by weight of Masterbatch made from 99% by weight of polyester    (SV of 800) and 1.0% by weight of Sylobloc 44 H (synthetic SiO₂ from    Grace), SPAN98=1.9

The production conditions in each of the steps of the process were:

The production conditions in each of the steps of the process were:Extrusion Temperatures Layer A: 270 ° C. Layer B: 290 ° C. Layer C: 290° C. Temperature of take-off roller 30 ° C. Longitudinal Heatingtemperature 80-125 ° C. stretching Stretching temperature 122 ° C.Longitudinal stretching ratio 4.2 Transverse Heating temperature 100 °C. stretching Stretching temperature 135 ° C. Transverse stretchingratio 4 Setting Temperature 230 ° C. Duration 3 s

The film had the required good sealing properties and the desiredhandling properties, and the desired processing performance, inparticular the processing performance on systems which operate in avacuum. To assess this performance, the film was coated on acommercially available vacuum-coating system (Top Beam) fromApplied/Hanau, using SiO_(x) with standard parameters. The thickness ofthe SiO_(x) layer was 200 nm and the coating speed was 300 m/min. Thestructure of the film and the properties achieved in films produced inthis way are shown in tables 2 and 3.

EXAMPLE 2

Using example 1 as a basis, the outer layer thickness for the sealablelayer (A) was raised from 1.5 to 2.5 μm, while the structure of the filmand the method of production were otherwise identical. There was aresultant improvement in sealing properties, and in particular a markedincrease in seal seam strength. The film had the required sealingproperties and the desired handling properties, and the desiredprocessing performance on systems which operate in a vacuum. The filmwas coated in a vacuum as in example 1.

EXAMPLE 3

Using example 1 as a basis, a film of thickness 20 μm was now produced.The outer layer thickness for the sealable layer (A) was 3.0 μm, andthat for the non-sealable layer (C) was 2.0 μm. Again, there was aresultant improvement in sealing properties, in particular a markedincrease in seal seam strength. The film had the required sealingproperties and the desired handling properties, and the desiredprocessing performance on systems which operate in a vacuum. The filmwas coated in a vacuum as in example 1.

EXAMPLE 4

Using example 3 as a basis, the copolymer for the sealable outer layer(A) was changed. Instead of the amorphous copolyester having 78 mol % ofpolyethylene terephthalate and 22 mol % of ethylene isophthalate, usewas now made of an amorphous copolyester having 70 mol % of polyethyleneterephthalate and 30 mol % of ethylene isophthalate. The outer layerthickness for the sealable layer (A) was again 3 μm, and that for thenon-sealable layer (C) was 2.0 μm. There was a further improvement insealing properties, in particular a marked improvement in seal seamstrength. To achieve good handling properties and good processingperformance of the film, the concentration of particles in the two outerlayers was slightly raised. The film was coated in a vacuum as inexample 1.

Comparative Example 1

Using example 1 as a basis, an unpigmented sealable outer layer (A)(comprising no particles) was now used. Although there was a resultantimprovement in sealing properties, there was an unacceptabledeterioration in the handling properties of the film and the processingperformance. The film was coated in a vacuum as in example 1. Duringthis process severe blocking occurred between the coated side (C) andthe sealable side (A). Due to relatively large amounts of ceramictransfer (from the outer layer (C) to the outer layer (A)), furtherprocessing of the film was impossible.

Comparative Example 2

Example 1 from EP-A-0 035 835 was repeated. The sealing performance ofthe film, the handling properties of the film, and the processingperformance of the film are poorer than in the examples of theinvention. The film was coated in a vacuum as in example 1. In addition,the film had poor haze.

Comparative Example 3

Example 1 of EP-A-0 515 096 was repeated. The processing performance ofthe film in a vacuum is poorer than for the films of the invention. Thefilm was coated in a vacuum as in example 1. In addition, the haze ofthe film was unsatisfactory.

Comparative Example 4

Example 21 of EP-A-0 379 190 was repeated. The processing performance ofthe film in a vacuum is markedly poorer than for the examples of theinvention. The film was coated in a vacuum as in example 1.

TABLE 2 Film Median particle Particle thick- Film Layer thicknessesdiameter in concentrations Ex- ness struc- μm Particles in layers layersμm ppm ample μm ture A B C A B C A B C A B C E 1 12 ABC 1.5 9 1.5Sylysia 430 None Sylobloc 44 3.4 2.5 2000 0 1500 H E 2 12 ABC 2.5 8 1.5Sylysia 430 None Sylobloc 44 3.4 2.5 2000 0 1500 H E 3 20 ABC 3 15 2Sylysia 430 None Sylobloc 44 3.4 2.5 2000 0 1500 H E 4 20 ABC 3 15 2Sylysia 430 None Sylobloc 44 3.4 2.5 2500 0 2000 H CE 1 12 ABC 1.5 9 1.5None None Sylobloc 44 2.5 0 1500 H CE 2 15 AB 2.25 12.75 Gasil 35 None 32500 0 0 CE 3 15 AB 3 12 Aerosil None 0.04 8000 0 0 K330 CE 4 11.5 AB2.5 9 Silica None 2 5000 400 0

TABLE 3 Minimum Winding sealing Seal seam Coefficient of Averageperformance temperature strength friction roughness R_(n) Gloss andExam- ° C. N/15 mm COF nm Haze (20° ) handling Processing ples fin lapfin lap A/A C/C Side A Side C % Side A Side C properties performance E190 110 2.9 2.4 0.58 0.44 143 85 1.9 166 157 ++ ++ E2 90 106 4.2 2.7 0.650.44 123 85 2.1 160 180 ++ ++ E3 90 106 4.4 2.7 0.65 0.44 123 85 2.1 160180 ++ ++ E4 90 105 4.7 3.4 0.58 0.43 146 95 2.3 170 175 ++ ++ CE1 90110 3.2 2.4 >1 0.44 20 85 1.7 185 180 — — CE2 105 — 0.97 0.49 70 20 22.5140 — — CE3 103 — 4.125 0.63 >1 ≈25 20 2.7 140 — — CE4 100 — 4 0.65 0.8≈65 30 1.5 140 — — Key to winding performance, handling properties, andprocessing performance of films: ++: no tendency to stick to rollers orto other mechanical parts, no blocking problems on winding or duringprocessing on packaging machinery, low production costs +: moderateproduction costs −: tendency to stick to rollers or to other mechanicalparts, blocking problems on winding or during processing on packagingmachinery, high production costs due to complicated handling of the filmin the machinery

1. A coextruded, transparent, biaxially oriented, sealable polyesterfilm with at least one base layer (B), with a sealable outer layer (A)on one surface of the base layer (B), and with another outer layer (C)on the other surface of the base layer (B), where the sealable outerlayer (A) has a minimum sealing temperature (fin) of not more than 110°C. and a seal seam strength (fin) of at least 1.3 N/15 mm of film width,and wherein the two outer layers (A) and (C) are characterized by thefollowing features: Sealable outer layer (A): a) comprises particlescomposed of synthetic SiO₂ at a concentration of from about 500 to about5000 ppm, with a median particle diameter d₅₀ of from about 2.5 to about10 μm, the SPAN98 of the diameter being less than or equal to 1.80, b)has roughness R_(a) greater than 60 nm, c) has a side A/side Acoefficient of friction smaller than 0.8, and non-sealable outer layer(C): d) comprises particles composed of synthetic SiO₂ at aconcentration of from above 1000 to about 5000 ppm with a medianparticle diameter d₅₀ of from 1.5 to about 6 μm, the SPAN98 of thediameter being less than or equal to 2.0, e) has roughness R_(a) suchthat 30≦R_(a)≦150 nm, f) has a side C/side C coefficient of frictionsmaller than 0.6.
 2. The sealable polyester film as claimed in claim 1,wherein the sealable outer layer (A) comprises an amorphous copolyesterwhich is composed of ethylene terephthalate units and of ethyleneisophthalate units, and of ethylene glycol units.
 3. The sealablepolyester film as claimed in claim 2, wherein the amorphous copolyesterof the sealable outer layer (A) contains from about 40 to about 95 mol %of ethylene terephthalate and from about 60 to about 5 mol % of ethyleneisophthalate.
 4. The sealable polyester film as claimed in claim 1,wherein the thickness of the sealable outer layer (A) is greater than0.5 μm.
 5. The sealable polyester film as claimed in claim 1, whose hazeis less than 3.0.
 6. The sealable polyester film as claimed in claim 1,wherein the amount of particles in the base layer (B) is smaller than inthe two outer layers (A) and (C).
 7. A process for producing a sealablepolyester film as claimed in claim 1, in which the polymers for the baselayer (B) and the two outer layers (A) and (C) are fed to separateextruders, any foreign body or contamination present is removed from thepolymer melt by suitable filters prior to extrusion, and the melts arethen shaped within a coextrusion die to give flat melt films and arebrought in contact with one another to form layers, and then themultilayer film is drawn off with the aid of a chill roll and issolidified, and then is biaxially stretch-oriented and heat-set, thebiaxial stretching being carried out sequentially by first stretchinglongitudinally (in machine direction) and then transversely(perpendicularly to machine direction), which comprises carrying out thelongitudinal stretching at a temperature in the range from about 80 toabout 130° C. and the transverse stretching in the range from about 90to about 150° C., and comprises using a longitudinal stretching ratio inthe range from about 2.5:1 to about 6:1 and a transverse stretchingratio in the range from about 3.0:1 to about 5.0:1.
 8. The process asclaimed in claim 7, wherein, after the longitudinal stretching and priorto the transverse stretching, one or both surfaces of the film is or arecoated by the in-line method.
 9. Method of making a packaging film,which method comprises converting a film as claimed in claim 1 into apackaging film.